Supine push-ups - help please


Hi, all. I could use some form advice. For my life, I just can't seem to do supine push-ups properly. I even checked out the how-to video on Fitness Blender. When I do them, I either don't feel my upper back muscles working, or my shoulder joints hurt from over-trying.

Do you guys have any tricks for how to gauge whether you're doing them effectively? Any pointers would be much appreciated. Thanks!